The Eleocharis mamillata H. Lindb. fil. aggregate (Cyperaceae) in the British Isles

We analysed stylopodium characters, perianth bristle number, vascular bundle number, stomatal length and fruit density for 50 stems of Eleocharis mamillata agg. throughout its British range. Stylopodium shape varies considerably within a spikelet and it is necessary to measure a large sample of fruits to establish an accurate reading. However there is highly significant variation between individuals for stylopodium shape and relative stylopodium width. Variation in these attributes between individuals is continuous. Nine individuals sampled, eight from N.W. Yorks (v.c. 64) and one from Selkirk (v.c. 79), have a stylopodium shape considered typical of subsp. mamillata, which has not been previously recorded from the British Isles. However, this material does not correspond to subsp. mamillata in any other character. Most individuals from the Borders region (v.cc. 67 and 70) are clearly referable to subsp. austriaca (Hayek) Strandhede, but many individuals in N.W. Yorks, and one from Selkirk belong to a distinctive intermediate type which is also distinguished by a high fruit density and vascular bundle number. We hypothesise that these may represent hybrids between previously isolated populations which have been recently transported into water bodies by ducks.